Content packs can be downloaded from Autodesk Exchange for AutoCAD Plant 3D (App store) after installation. For more information see How to find Content Packs for AutoCAD Plant 3D html.
For information about content available during installation see: http://beehive.autodesk.com/community/service/rest/cloudhelp/resource/cloudhelpchannel/guidcrossbook/jsonp?v=2016&p=PLNT3D&l=ENU&guid=GUID-2FBCF9C4-BCDE-480E-B01D-1A358BEC7D17 .